1. Field of the Invention
The present invention relates generally to surgical instruments and methods, and is particularly concerned with an ophthalmic instrument and method in which a flexible wire is used to make a line of perforations in the anterior lens capsule of the eye.
2. Description of the Prior Art
Endophthalmic surgery, or surgery on an intact and normally pressurized eye, represents an important and relatively recent development in the field of ophthalmology. In this technique, the existing optical pathways of the pressurized ocular globe are utilized for visualization during delicate intraocular manipulations. Maintenance of positive intraocular pressure tends to preserve and stabilize the spatial relationships among the various intraocular tissues.
The archetypal endophthalmic procedure is the cataract aspiration technique of extracapsular cataract surgery described by Scheie, Am. J. Ophthal. 50:1048 (1960), wherein an instrument is passed through a small incision at the margin of the cornea into the anterior aqueous chamber of the eye to incise the anterior capsular membrane of the lens. The aqueous, which leaks out during this manipulation, is replaced by a gravity-fed infusion of physiologic saline through a cannula inserted into the anterior chamber through a second small incision. A blunt needle is then inserted through the first incision into the lens, whereupon gentle suction aspirates the soft lens substance leaving the posterior capsular membrane in place. Whatever volume is removed or leaks from the two small incisions is replaced by the continuous gravity feed of saline. Absent any seepage or applied suction, the pressure in the eye stabilizes at a point determined by the physical elevation of the saline column above the level of the eye. At the end of the procedure, all tubes are withdrawn and the incisions are sutured.
The crystalline lens of the eye is a viscid, cellular, biconvex structure enclosed in a transparent, elastic membrane known as the capsule. The lens adjoins the posterior iris and posterior chamber anteriorly, the zonular ligaments and ciliary body equatorially, and the vitreous body posteriorly. When disease or degeneration degrades the optical quality of the crystalline lens, the resulting opacity is called a cataract. Modern cataract surgery involves removal of either the entire lens and capsule, which is known as intracapsular cataract extraction (ICCE), or piecemeal removal of the lens substance after opening or excising the capsule, which is known as extracapsular cataract extraction (ECCE). Both techniques have undergone steady refinement over the past century in order to reduce the risks and improve the results of cataract surgery.
Modern endophthalmic ECCE requires a controlled, predictable opening in the anterior capsule. Many techniques and instruments have been proposed with which to accomplish this.
The earliest method of opening the anterior capsule was needling or incising the membrane with multiple, slashing incisions made with a knife needle or a scythe-like instrument called a cystotome. Another early technique involved grasping the anterior capsule with tooth forceps and tearing off a piece of the capsule. Of course, because of the large size of the instrument, it was necessary to open the eye in order to maneuver the instrument onto the surface of the capsule.
The precursor of modern capsulectomy techniques is the "Christmas tree" or dull cystotome method as popularized by Kelman. The anterior capsule is engaged opposite the entry site and is torn in one movement toward the surgeon, creating a triangular flap which is pulled out of the eye and then excised. As originally described by Kelman, other small tears could be added along the sides of the triangle to enlarge the opening.
Renewed interest in ECCE in recent years has led to many alternative capsulectomy methods. The common theme has been to increase the control of the excision with less emphasis on tearing and ripping. Also, it is well recognized that endosurgical capsulectomy is particularly desirable in order to visualize the capsule and avoid damage to adjacent intraocular tissues.
One technique suggested the use of a circular resistance wire encased, except for its concave posterior surface, in an insulating disc which would be apposed to the anterior capsule with mild suction. The wire would be briefly heated to cut the membrane. This technique has not become commercially available.
The most widely used capsulectomy technique at the present time is a modification of the "Christmas tree" method, known as the "canopener" method. The instrument used is usually a 25 to 30-gauge hypodermic needle with its bevel bent at 90 degrees to the shaft. This is placed in the eye through a small incision and used to make small triangular tears, which are typically confluent, along the desired line of excision. Although in skilled hands this is an extremely effective method, it is difficult to master for the occasional ECCE surgeon. Another drawback is that the pupil must be widely dilated, since the instrument is controlled by direct visualization. This technique is also very difficult when the capsule is very thick and loose, as in traumatic cataract, or when the capsule is thin and tense, as in a mature or ripe cataract.
There has been at least one attempt to design a powered surgical instrument capable of opening the anterior lens capsule of the eye prior to cataract surgery. In U.S. Pat. No. 3,809,093 to Abraham, a hand-held surgical instrument is described which includes a rod or probe terminating in a small globular tip. An electromagnetic vibrating mechanism in the handle portion of the instrument imparts a limited transverse arcuate or swinging motion to the rod and tip at a controllable rate up to about 100 cycles per second. The globular tip may be provided with a pointed or knife edged projection in order to form multiple incisions in the anterior capsule of the lens as the rod or probe vibrates. With different types of smooth, abrasive or knife-edged globular tips, the instrument can also be used to carry out other types of intraocular surgical procedures, such as cataract disintegration or removal of the posterior lens capsule. The difficulty with this instrument, at least insofar as its application to removal of the anterior lens capsule is concerned, is that the vibration of the globular tip occurs in a direction transverse to the direction of the pointed or knife-edged projection. As a result, the incisions in the anterior lens capsule are actually small rips or tears, rather than discrete perforations. Also, the exposed vibrating tip of this instrument may pose a danger to neighboring intraocular tissues, particularly when it is desired to incise the anterior lens capsule in the marginal region beneath the dilated iris.
More recently, pulsed infrared and near-infrared lasers have been demonstrated to be capable of creating multiple fine perforations of the anterior capsule prior to surgery. This development has been reported by D. Aron-Rosa, Am. Intra-Ocular Implant Soc. J. 7:332 (1981). The laser method is advantageous because it requires no direct instrument contact with the lens capsule, and hence it is substantially non-invasive. However, this method requires a clear optical path to the capsule surface and is therefore incapable of forming the desired line of perforations along the marginal portion of the anterior capsule which is obscured by the dilated iris. Another drawback of this technique is that the necessary laser equipment is extremely large and extraordinarily expensive, and accomplished essentially the same result that can be obtained by using a disposable hypodermic needle.
In summary, the evolution of eye surgery in general and ECCE in particular is characterized by emphasis on precise placement of all tissue handling instruments, controlled incisions rather than unpredictable ripping or tearing, and the use of endosurgical techniques allowing microscopic visualization using the intact optical pathways of the eye.